In tractors, a change in tire pressure is especially important. When working in the field, a lower tire pressure provides a lower ground compaction and higher traction. At high speeds, e.g. 40 km/h, tractors with low tire air pressure generally have a driving instability. In the trade paper, profi-magazin for agricultural technology, October/1995 “The air pressure in the tire: Small causes, large effects” different regulating devices are found. According to this, the rotary feedthroughs are arranged on the wheel flange according to the axle type either on the inner side or the outer side of the wheel. In less expensive systems, the air feed is carried out passed the outside of the tire up to the rotary feedthrough. In an arrangement with an intermediate flange between the wheel and the wheel flange the attachment of the rotary feedthrough is achieved via the wheel screws. This embodiment has an advantage since conduits are not exposed to the outside. However, the critical screw connection is loaded even higher by the enlarged free bending length of the wheel screws.
U.S. Pat. No. 4,804,027 describes a device for inflating and deflating a tire on a wheel of a vehicle fillable with air. The device is integrated into the wheel hub, which supports the wheel flange. The wheel hub forms the outer member with a channel, to which a connection line is connectable, which leads to the tire. An inner member is supported in the hub. The inner member is kept stationary and is connected to a pressure source. Two seals are non-rotatably arranged on the inner member and distanced from each other. The two seals together with a valve arrangement enclose an annular chamber. The valve arrangement includes a spring loaded valve. The valve is opened when a predetermined pressure is exceeded in the chamber. This allows the air to flow into the channel of the wheel hub. The valve, however, is formed so that it only opens when the seals abut the corresponding bore face of the wheel hub lying radially to the outside. The seals are always pressed against the corresponding counter face of the wheel hub with the total pressure that is present to inflate the tire. At high relative speeds this is disadvantageous. At high speeds, the high pressure leads to a high friction and, therefore, also to a corresponding high wear of the seals. A further disadvantage is that the hub and the inner member are supported independently from each other on a further component. Thus, radial deviations completely act on the seals.
DE OS 1 605 743 describes a device that controls the tire pressure in vehicles. It describes a rotary feedthrough with an outer member and an inner member. The outer and inner members are rotatably sealed relative to each other and are sealed with contact seals towards each other and constantly carry out a relative movement.
A tire inflating device is also described in DE-OS 1 907 082. This reference discloses that it is necessary for vehicles with relative high circumferential speeds, to switch off the rotary feedthrough during normal operation. Accordingly, during the non-actuation of the tire inflating device, no pressure is present. This eliminates problems that arise at the seal elements of the rotary connection. This problem is especially obvious, when large relative speeds are present, as is the case in the usage in tractors.
In all the arrangements described above and belonging to the State of the Art, the seals, which are formed as soft seals, have a high wear at their seal lips, which leads to a short life span.